ICPT is now a widely used technique preferred for the transfer of power in situations such as clean rooms where dirt and dust must be avoided, or situations such as manufacturing plants where dirt and dust must be tolerated. Such systems take power from a utility or “mains” power supply and convert that power to a current in a conductive path which is typically an elongated loop commonly referred to as a track. Pick-up coils along the track intercept some of the magnetic field from the track. These coils are invariably tuned or compensated with a capacitor and in this way the power transferred from the track to the pick-up coil may be augmented. The power transfer may be controlled using the technique described in U.S. Pat. No. 5,293,308, the contents of which are incorporated herein by reference. The power transferred to the pick-up coil is rectified and controlled to produce a constant voltage DC output from which other devices may be operated. Alternatively a controlled output current could also be produced, but this would be a less preferred option.
In known ICPT systems the current in the track has been essentially a constant current at a VLF frequency typically in the range 5-50 kHz. Lower frequencies are not preferred as the power transfer process is more difficult, and higher frequencies are also avoided as the inductance of the track makes the establishment of a track current more difficult as the frequency increases, and the voltage required to achieve the required current increases.
To generate a constant current in the track, a conventional method is to rectify a 3-phase input from the utility supply to produce an essentially constant DC voltage and then use an inverter to produce a high frequency voltage or current signal that may then be applied to the track. In this way there are great advantages to using a 3-phase power supply as the input currents are then at an acceptable power factor and the output voltage and current are essentially constant as required. However, this technique requires two energy storage elements—a DC capacitor and a DC inductor—both of which are relatively expensive and bulky. Also as there may be significant energy stored in these elements, circuitry to control this energy particularly when switching on or off is essential. This circuitry adds cost and takes extra space. These elements store energy and operate at high voltages and currents so that micro-miniaturisation is not possible. It is also time consuming and inefficient to turn such known supplies on and off, so they tend to remain energised which adds significantly to overall operating costs.
A resonant converter that operates form a single phase supply has been proposed in “Characteristics of Load Resonant Converters Operated in a High-Power Factor Mode” by Schutten et al in IEEE Transactions on Power Electronics, Vol. 7. No. 2, April 1992. However the supply, being an LCC converter, is inherently unsuitable for use with an ICPT system. Also, improvements in power factor are obtained by control of the power supply which makes the supply more complex, and is unsuitable for multiple varying loads such as those typically present in many ICPT applications.
Object
It is an object of the present invention to provide an ICPT system, or a power supply for an ICPT system which overcomes or ameliorates one or more of the disadvantages of known systems.
Alternatively it is an object of the present invention to provide a single-phase power supply for an ICPT system.
Alternatively it is an object of the present invention to provide an ICPT system, or a method of operating such as system, or a power supply for an ICPT system, which will at least provide a useful alternative to known systems.